1. Field of the Invention:
The present invention relates to a power transmission apparatus, and more particularly to a hydraulic power transmission apparatus comprising an oil pump and an oil motor for changing the speed of rotation of an output shaft.
2. Description of the Relevant Art:
Various vehicles such as lawn mowers, tractors, and the like have power transmission apparati for reducing the speed of rotation of an output from a power source such as an engine, an electric motor, or the like and also for transmitting the power to the axles of drive wheels while changing the speed of rotation at a suitable transmission ratio.
One type of such power transmission apparatus comprises hydraulic devices such as an oil pump and an oil motor. The oil pump and the oil motor serve to change the speed of rotation at a desired transmission ratio, whereas the speed of rotation is reduced by a speed reducer mechanism coupling a plurality of gears.
Such a hydraulic power transmission apparatus is disclosed in Japanese Laid-Open Utility Model Publication No. 61-19158, for example. In the disclosed hydraulic power transmission apparatus, the oil pump and the oil motor are housed in a case which is separate from a transmission case housing the speed reducer mechanism.
Since the oil pump and the oil motor are disposed separately from the transmission case which accommodates the speed reducer mechanism, the power transmission apparatus is large in size, and the case is complex in shape and highly costly.
The disclosed power transmission apparatus also has two attachment plates. The case housing the oil pump and the oil motor is attached to one of the attachment plates, whereas an oil charge pump is assembled in the other attachment plate to which the transmission case housing the speed reducer mechanism is secured, the attachment plates being fastened to each other.
Oil passages interconnecting the oil pump, the oil motor, and the oil charge pump are defined in both of the attachment plates, and a pipe is employed to connect the oil charge pump and an oil reservoir.
The power transmission apparatus further includes an on/off valve for returning oil discharged from the oil pump to an oil tank to free the drive wheels.
However, with the on/off valve assembled in a base plate, the base plate must have a small hole in which the valve body of the on/off valve is inserted.
Japanese Laid-Open Utility Model Publication No. 60-37654 discloses another power transmission apparatus in which an oil pump and an oil motor are housed in a single case which has a breather mechanism for venting the space in the case to atmosphere.
According to another conventional apparatus disclosed in Japanese Patent Publication No. 38-7172, the drive shaft of an oil pump and a power source are coupled to each other by a power transmission shaft.
Japanese Patent Publication No. 49-2337 discloses another power transmission apparatus in which an oil pump and two oil motors are provided and a pair of speed reducer mechanisms are disposed at the output shafts of the oil motors, respectively.